1. Field of the Invention
The present invention relates to a process for retorting a carbon containing solid such as oil shale. More particularly, the process pertains to a combination staged turbulent bed/moving packed bed retort.
2. Description of the Prior Art
Certain naturally-occurring materials such as oil shale and tar sands, including diatomite, contain a carbonaceous fraction which during retorting releases an oil useful in petroleum processing.
In a staged turbulent bed retort, crushed raw oil shale particles (or other carbon containing solids) and particulate heat transfer materials at an elevated temperature, such as hot burned shale, are introduced into an upper portion of a vertical retort and pass downwardly therethrough. A strippling gas, preferably steam, is introduced into a lower portion of the retort and passes upwardly through the vessel in countercurrent flow to the downwardly moving solids.
The maximum particle size for the solids introduced is preferably maintained at or below 21/2 mesh. Tyler standard sieve size. Oil shale sizes in this range are easily produced by conventional means such as combinations of cage mills, jaw or gyratory crushers. The crushing operations may be conducted to produce a maximum particle size, but little or no control is effected over the smaller particle sizes produced, and thus a broad particle size distribution is encountered.
With such broad particle size distributions a portion of the solids is fluidized and the remaining solids are either entrained and swept out of the system or pass to the bottom of the retort. Stable fluidization conditions are maintained by substantially limiting gross vertical backmixing and slugging of the solids through the use of baffles disposed throughout the retort. Typical baffles suitable for use include perforated plates or grille structures horizontally disposed in the retort vessel.
A detailed description of the staged turbulent bed retort may be found in U.S. Pat. No. 4,199,432, issued to Paul W. Tamm and Gordon Langlois on Apr. 22, 1978, which patent is incorporated herein by reference.
Although the staged turbulent bed retort is capable of handling large mass flow rates, the retort vessel must be sufficiently high to provide a residence time of approximately 2 to 10 minutes to allow the kerogen pyrolysis reaction to occur. In contrast to the relatively long residence time required for the reaction, heat transfer between heat carrier particles and raw shale particles requires only a residence time of 15 to 50 seconds for the largest particles. Although the staged turbulent bed is ideal for heat transfer purposes, its vigorous mixing action results in attrition of the retorted shale particles and subsequent carry over of fines from the retort vessel by the vapor point.
Thus, it becomes desirable to reduce the residence time of particles in the staged turbulent bed to a minimum, namely that required for heat transfer, and provide residence time for the kerogen pyrolysis reaction in a less turbulent environment.